1. Field of the Invention
The present invention relates to backside defect detectors and, more particularly, to a backside defect detector and method that utilizes an IR light scan to determine whether unwanted materials are present on the backside of a semiconductor wafer.
2. Description of the Related Art
During a conventional semiconductor wafer fabrication process, a number of materials are formed on the surface of a semiconductor wafer, and then selectively removed during a series of steps that form a structure which operates as an electrical circuit. Typically, the materials that are formed on the front side of the wafer are also formed on the backside of the wafer.
One well-known and common fabrication step is photolithography. In photolithography, a photoresist is formed on the top surface of a wafer. The photoresist is then patterned, developed, baked, and partially removed to form a mask that exposes regions of an underlying material. In most cases, the photoresist has an etch rate that is substantially different from the etch rates of the exposed materials that underlie the mask.
After the mask has been formed, the next fabrication step is typically to etch the exposed materials using a wet and/or dry etchant. The etchant removes the materials that are exposed by the mask over a period of time due to the difference in etch rates between the photoresist and the exposed materials.
The backside of the wafer is often exposed so that a wet etchant can be used to remove the materials that were formed on the backside of the wafer. Ideally, the wet etchant removes all of the materials that were formed on the backside of the wafer so that the backside of the wafer is clean and flat.
In actual practice, however, after the etch step has been completed, minute amounts of materials can undesirably remain on the backside of the semiconductor wafer. The minute amounts of material that remain on the backside of the wafer, in turn, can distort the flatness of the backside of the wafer.
As a result, the next time the backside of the wafer is placed on a flat surface for processing, the distorted backside causes the top surface of the wafer to also be distorted. In other words, locations across the top surface of the wafer which are intended to have the same height can have different heights when measured normal to the flat surface.
Thus, when a photoresist is subsequently deposited across the top surface of the wafer, the photoresist does not have a uniform depth in each of the places where the depth should be uniform. The lack of uniform depth leads to focus spots that alter the image pattern that is formed in the photoresist.
As a result, non-uniform opening sizes can be formed when the photoresist is subsequently developed, baked, and removed to form a mask. In sub-micron applications, these non-uniform opening sizes can lead to improperly sized devices that fail to meet specification or fail to operate all together.
Therefore, to reduce the photolithographic distortion (the focus spots) that are due to minute materials that undesirably remain on the backside of the wafer, and the resulting non-uniform opening sizes, there is a need for an apparatus and method that determines whether unwanted materials are present on the backside of a semiconductor wafer following a processing step, and therefore whether the backside of the wafer is clean and flat and ready for a subsequent processing step.